This invention relates to photoelectric detector circuits and controls for those circuits, and in particular to those circuits as applied to movable storage units to disable the movement of those units if there is something in the way.
Movable storage units are of course well known, as indicated in many U.S. Patents such as Dahnert, U.S. Pat. No. 4,557,534. As described in that patent, some examples of movable storage units are library bookshelves, file cabinets, film storage files and racks used in warehouses and industry to store parts and finished and unfinished goods. Most commonly the storage units are mounted on wheeled carriages which run on rails. Each carriage normally has at least one prime mover such as a reversible electric motor for propelling it along the rails. In order to avoid damaging an object or injuring a person in the way of the moving carriage, it has been conventional to provide the carriage with a sweep bar that sweeps along the floor preceding the moving carriage to actuate a safety switch when the bar contacts the object so as to arrest the movement of the carriage. Depending upon the nature of the object, however, contact between the object and the sweep bar can damage the sweep bar. In addition, the sweep bar can be subject to damage from other sources. For instance, if the storage units are used in an industrial application, there is potential for damage from material handling equipment such as lift trucks.
One alternative to a mechanical sweep is the use of a photoelectric detector device, to detect the existence of an obstructing object in the path of the moving carriage and then disable the movement of the carriage. The problem here is that, since the application is critical, involving as it does the personal safety of users of the movable storage units, the detection system must be made fail-safe.
This invention relates to improvements to the apparatus described above and to solutions to the problems raised thereby.